Composite tidal turbine blades with bend-twist (BT) coupled layups allow the blade to self-adapt to local site conditions by passively twisting to reduce the angle of incoming flow (feathering). Passive feathering has the potential to reduce the fluid forces on both the blades and support structure, as well as shed power at extreme site conditions. Decreased hydrodynamic thrust and power at extreme conditions means that the turbine support structure, generator, and other components can be sized appropriately for rated conditions, increasing their utilization factor and increasing the device cost effectiveness.
This thesis reports the outcomes of research into passively adaptive BT blades. A design tool was developed that couples a finite element model (FEM) and a blade element momentum theory (BEMT) model, to investigate the interactive fluid and structural performance of BT blades. The design tool also incorporated a composite material failure analysis, allowing fast and efficient verification of the structural integrity of different blade designs. Through experimental testing of blades designed using the tool, BT composite blades were shown to have up to 10% lower thrust loads compared to rigid blades, with similar load reductions predicted by the design tool. This proved the concept and demonstrated a design methodology for BT coupling for tidal turbine blades at small-scale.
A case study of a full-scale turbine with 4.0 m BT blades with a pre-deformed blade shape (slightly decreased pre-twist distribution along the blade span) was investigated using the design tool. By reducing the pre-twist of the blade by 2.3º at the blade tip, the blade twisted under load to its optimum shape at design conditions, and continued to twist to feather toward extreme flow speeds. These blades were found to have 10% more power capture between the cut-in and design speeds, and a 10% reduction in power and 5% reduction in thrust loads at extreme flow speeds. This makes pre-deformed BT blades a potential solution to structural load reduction, as well as power capture optimization, which would increase the overall cost-effectiveness of the tidal turbine.